This invention relates generally to hot-air ovens for reheating packages containing pre-cooked meals initially in a frozen state, and more particularly to a counter-top unit of this type which is useable in homes and offices and is adapted rapidly to reheat a group of packages to a service temperature level and to maintain the meals at this level for an indefinite period.
To satisfy the growing need for quickly prepared inexpensive meals, convenience food systems have been developed in which the meals to be served at a later time are first cooked and packaged, and then deep-freezed. When one wishes to eat a particular meal, the selected package is taken out of the freezer and the frozen pre-cooked meal is then thawed and reheated. Typical of such operations is the so-called TV dinner in which a pre-cooked meal in the frozen state is sealed within a serving tray, the dinner being kept in the freezer until there is a demand for it, at which point the TV dinner is thawed and reheated in a microwave oven, a convection oven or whatever heater is available.
The term "packages" as used herein is intended to cover any sealed dish, tray or other container having a pre-cooked meal therein.
In reheating a pre-cooked frozen meal in homes and offices, it is difficult with conventional hot air ovens, when going from the frozen state to a service level, to avoid a situation in which the core of the meal is still cold even though the outer layer is quite hot. When one seeks to ensure that the body of the food is hot throughout, there is a tendency to overheat the meal and thereby re-cook it, with a resultant loss in nutritional value and flavor. And assuming that the meal has been heated to a proper serving level, it must be served without delay, for with the typical oven it is virtually impossible to thereafter hold the meal in the oven until such time as there is a demand therefor without overheating.
A microwave oven has the advantage of reheating a pre-cooked meal from the inside out, rather than the other way around. But since microwave heating depends on the dielectric properties of the body being heated and no two pre-cooked frozen meals have the same characteristics in this regard, the results of microwave heating are highly variable. Moreover, once the pre-cooked meal is heated in a microwave oven, it cannot be maintained in a heated condition and must be served shortly thereafter.
For a convenience food operation to be effective, one must be able not only to reheat the pre-cooked meal to a proper service temperature level within a relatively short time, but one must be able to take into account the fact that in a home and office, the time at which diners are ready to eat may be subject to change. Thus in a typical office having several staff members, all of whom intend to lunch at say noon, it is not at all unusual for one or more of the members to be unavailable until say an hour or so later. Existing ovens for reheating precooked frozen meals cannot cope with this common contingency.
My prior U.S. Pat. No. 4,112,916, "Hot Air Oven for Food-Loaded Cartridges" discloses a fast food service technique in which pre-cooked food which has been refrigerated may thereafter be reheated and made directly available to diners without degrading the essential texture, flavor or nutritional qualities of the meal.
In this patent, a hot air oven is provided for heating tray-loaded cartridges, each constituted by a stack of sealed trays containing pre-cooked meals nested within an open carton whose side walls have holes therein to admit heated air. The oven includes a rotating turntable provided with a raised annular shelf for supporting an annular array of cartridges, the side walls of which define a hollow center core.
A heater assembly above the annular cartridge array produces heated air which is blown into the hollow core. Because of the flow restriction, a substantial portion of the heated air is forced through the holes of the cartons to heat the food in the trays. The remaining portion of the heated air passes through the flow passage, the air discharged from the outlet thereof being drawn upwardly by suction force to create an air curtain around the cartridge array.
In an oven of the type disclosed in my prior patent, a two-section heating assembly is provided having different wattages, whereby at the outset of heating, both sections are operative for a controllable period, hereinafter called the heat-up phase, sufficient to raise the food temperature to the desired service level, after which the main section is rendered inactive while the thermostatically-controlled auxiliary section, which draws much less power, then serves to maintain indefinitely the heated food at the proper level for service to diners, hereinafter called the "service phase."
During the heat-up phase, the rate of heat transfer from the hot air in the oven to the cold food-loaded cartridges depends on the temperature differential; the greater the difference between the hot air temperature and the food temperature, the more rapid the rate of heat transfer. Since the hot-air temperature throughout the oven is at a fairly uniform level, the transfer rate at the outset of the heat-up phase, when both heater sections are operative, is very rapid. But as the difference in temperature between the hot air and the food thereafter diminishes, the rate of transfer becomes increasingly slow as the service temperature is approached.
Assuming that the food in the cartridges is initially at a temperature of about 10.degree. F. and it is necessary to raise the food temperature to a service level of about 150.degree. F., and further assuming a hot air temperature of about 165.degree. F., then at the outset of the heat-up phase, there will be a sharp differential giving rise to very rapid heating. But as this temperature differential diminishes in the course of the heat-up phase, the rate of heat transfer slows down considerably. When, for example, the food temperature reaches 130.degree. F., the temperature differential relative to the heated air is only 35.degree. F., and it takes a relatively long time before the food temperature can be raised to the service temperature of 150.degree. F., at which point the heat-up phase is concluded and the service phase takes over with only one heater section operative to maintain this service temperature level.
Similarly, in convection-heating units of the type heretofore available commercially, one can set the oven for a desired heat-up temperature. But as previously explained, the temperature differential between the cold food and the heated air is large only in the initial heat-up period, and the closer the food approaches the service temperature, the smaller the differential and the more sluggish the rate of heat transfer. Consequently, it takes an unacceptably long time for the food to reach the service temperature. This is particularly the case when the unit is fully loaded with several trays or packages of frozen food.
If the operator of a standard oven tries to accelerate the heat-up phase by setting the temperature level of the oven well above the service temperature, the resultant heating will generally be destructive of the food; for the outer layers of the body of food will then be heated to an excessive level, causing these layers to be re-cooked or burned while the intermediate layers and the core of the body are still well below the service temperature level.
In my above-identified copending application, there is disclosed an oven adapted to rapidly raise the temperature of pre-cooked meals from the cold or frozen state to a service temperature level at which the meal is in condition to be service, and to maintain the meal at this level until there is a demand therefor. This oven is provided a thermally-insulated chamber having a fluid-permeable receiving compartment flanked by input and output plenums.
A main flow loop is provided in which the chamber is connected in a continuous flow path in series with a heater station and an air pump or blower in an arrangement in which air drawn via an output line from the output plenum and creating a negative pressure therein is conducted through the heater station and then forced in the heated state through an input line leading into the input plenum to create a positive pressure therein. The resultant pressure differential between the plenum causes heated air to flow at high velocity through the compartment to heat the meals contained therein.
A by-pass extending between the input to the heater station and the junction of the chamber and the pump in the main flow loop defines a feedback flow loop which excludes the chamber. A damper mechanism at this junction is cyclically driven to periodically block the flow of heated air through the main loop into the chamber and to divert the flow into the feedback loop for recirculation therein.
As a consequence, main loop flow through the chamber assumes the form of a pulsatory wave whose fluidic pulses have a peak temperature whose level is well above the predetermined temperature level and whose relaxation periods are at a temperature below this predetermined level, thereby promoting rapid heat transfer in the body of the pre-cooked meal without, however, raising the surface temperature thereof above this level. This action is continued until the entire body of the meal is at the desired service temperature level, at which point the system is operated to maintain this level indefinitely without overheating the meal.